PDS include a various number of ground and space-based navigational systems. Ground-based systems, such as the OMEGA navigation system, the Russian Alpha system, the Long Range Navigation (LORAN) system, and the like each use terrestrial radio signals for establishing position. PDSs also include satellite positioning systems (SPSs) and Global Navigation Satellite Systems (GNSS), such as Global Positioning System (GPS), Galileo, Glonass and BeiDou. SPSs, such as GPS, use a constellation of between 24 and 32 medium Earth orbit satellites that transmit precise radio frequency (RF) signals that allow SPS receivers to determine their current location, the time, and their velocity. An SPS receiver follows a positioning process to calculate its position by carefully timing the signals sent by three and preferably four or more of the constellation of SPS satellites.
Each satellite continually transmits messages containing the time the message was sent, a precise orbit for the satellite sending the message, i.e., the ephemeris, and the general system health and rough orbits of all SPS satellites, i.e., the almanac. These signals essentially travel at the speed of light through space and slightly slower through the Earth's atmosphere. The timing of each signal can be used to calculate the distance to each satellite thereby establishing that the SPS receiver is approximately on the surfaces of spheres centered at each satellite. The SPS receiver also uses, when appropriate, the knowledge that the SPS receiver is on or near the surface of a sphere representative of the Earth. This information is then used to estimate the position of the SPS receiver as the intersection of sphere surfaces. The resulting coordinates are converted to a more convenient form for the user such as latitude and longitude, or location on a map, and then displayed.
However, if the SPS timing measurements are used alone, these calculations can only occur after the SPS receiver has acquired the satellite signals of the three, or preferably four or more SPS satellites at the beginning of the positioning process. The length of time that it takes the receiver to acquire the satellite signals and fix the position is directly related to how much information the SPS receiver has before beginning the acquisition process. In standalone SPS, this acquisition and positioning process can be quite long during a cold start operation, during which the SPS receiver does not have much, if any, prior location information. This generally occurs the first time that a user turns on an SPS receiver. A cold start may be turned into a warm start by providing certain bits of information including one or more of the receiver approximate location, SPS time, almanac, ephemeris data, and the like. The less information the receiver has, the larger the search space, and the longer the acquisition and positioning time. Conversely, the more information the receiver has, the smaller the search space, and the shorter the acquisition and positioning time.
A seed location is an approximate location of the GPS/SPS receiver that is utilized, in conjunction with appropriate ephemeris and/or almanac information to determine satellite search windows. One common source of a seed location is from the most recent fix. This method assumes that the receiver has not dramatically changed its location. With this method, a SPS receiver will calculate its location and store that location information in memory for future use as a seed location. If, however, a seed location is used that is no longer accurate, the initial search windows used by the receiver will be inaccurate, resulting potentially in a failed initial search and in longer overall fix times. For example, a user in Dallas, Tex. uses and operates a SPS receiver in the Dallas area. The receiver stores location information indicating the Dallas area. The user turns the SPS receiver off and flies to San Diego, Calif. The user restarts the SPS receiver in San Diego and, during the SPS positioning process, the SPS receiver may utilize the stored seed location information from the local memory resulting is a false seed location and a failed initial search. When the seed location information is substantially wrong, the SPS receiver will determine the initial search windows based on this erroneous information and may fail to find the satellites within their respective Doppler and time offset windows. In this situation, the SPS receiver searches until the search times out, which could be a relatively substantial amount of time. After timing out, the receiver deletes the incorrect location information and will then perform a cold start, typically with much larger search window assumptions, which adds significant time to the acquisition and positioning process.
Because of the usefulness of a seed location to SPS receivers, technology has been developed for the receiver to actively obtain a seed location from various external sources, such as a cell tower or cell site or a short range wireless transmitter or the like. For purposes of this application, a short range wireless transmitter can be any number of RF transmitters, such as used in Bluetooth Special Interest Group's BLUETOOTH™ transmitters, Wi-Fi Aliance's WIFI™ or similar transmitters using the IEEE 802.11x wireless local area network (WLAN) protocols, femtocells, transponder devices, and similar devices. A SPS receiver having such technology may actively communicate with a cell site or wireless access point and either receive a seed location directly from the source or access a database to determine the location of the particular cell site or access point. However, the information obtainable from these kinds of sources may not always be reliable. For example, a company providing a wireless access point at a registered location may move the location of the wireless access point without updating the registered location. Therefore, if the SPS receiver polls the access point and looks up the location registered to that access point, the seed location would be incorrect. Similarly, should a SPS receiver happen across a cellular test site, any information received from that cell location would generally be unreliable as testing facilities do not typically spend the time and effort to set the location parameters. In other cases, the location may have never been set or may have been accidentally entered as an erroneous value. Using an erroneous seed position may result in a failed initial search and substantial delays to the overall location attempt.